Stirrer and cover for stirring corrosive charges

ABSTRACT

The stirring shaft of a stirrer which is used for stirring charges in a corrosion resistant, high pressure reaction tank is subjected to corrosion by the charge material. A plastic cover is provided which covers that portion of the stirring shaft which contacts the charge material and protects it against corrosion. The cover is firmly secured at one end to the shaft at a portion thereof which does not contact the charge material, and at its other end between the stirring shaft and a stirring element to which the stirring shaft is mounted.

United States Patent Kupka 1 Nov. 25, 1975 [5 1 STIRRER AND COVER FORSTIRRING 3,246,882 4/1966 Clough 259/108 x CORROSIVE CHARGES 3,622,12911/1971 Mazowski Inventor. Dieter Kupka, Binger Landstrasse 37a, 657Kira, Nahe, Germany Filed: Feb. 16, 1973 Appl. No.: 333,002

Foreign Application Priority Data Feb, 16. 1972 German 2207207 11.5. C123/252 A; 259/107; 285/292 Int. Cl.'- BOIF 7/16 Field of Search 23/252A, 285, 289, 290; 259/107, 108, 122; 285/292, 298, DIG. l0; 403/1, 50

[56] References Cited UNITED STATES PATENTS 2.958.517 11/1960 Harker ctal 23/259 X Primary E.\'uminerMorris O. Wolk Assistant E.\'mninerTimothyW. Hagan Attorney, Agent, or FirmEdwin E. Greigg 1 1 ABSTRACT Thestirring shaft of a stirrer which is used for stirring charges in acorrosion resistant, high pressure reaction tank is subjected tocorrosion by the charge material. A plastic cover is provided whichcovers that portion of the stirring shaft which contacts the chargematerial and protects it against Corrosion. The cover is firmly securedat one end to the shaft at a portion thereof which does not contact thecharge material, and at its other end between the stirring shaft and astirring element to which the stirring shaft is mounted.

20 Claims, 5 Drawing Figures US. Patent Novizs, 1975 Sheet20f2 3,922,146

STIRRER AND COVER FOR STIRRING CORROSIVE CHARGES BACKGROUND OF THEINVENTION The present invention relates to a stirrer with a plasticstirring member for stirring systems with a corrosion resistant lining,and especially for stirring charges in glasslined, high pressurereaction tanks, and more particularly to a stirrer in which the stirringmember is fabricated as a one-piece element having several arms, withthe stirring member being attached to one end of a metallic stirringshaft, which is provided with a plastic cover along that portion of theshaft which contacts the charge material.

For stirring charge material having heavy acid or lye concentrations,for example, the chemical industry uses corrosion resistant stirringtanks made of glasslined steel. Since the stirring elements operating inthese stirring tanks must also be corrosion resistant, they are enameledjust like the tanks.

During molding of the stirring elements, care must be taken that auniform and solidly adhering enamel coating develops everywhere. As aresult, those stirring elements which are enameled cannot be given themost favorable shape insofar as flow and stirring techniques areconcerned. This is particularly true of propellerlike stirring elementswhere it has been noted that the stresses at the ends of the stirringarms produce vibrations which lead to chipping off of the enamel layer.This result is particularly pronounced with large dimension stirringelements. Also, enameled stirring elements can no longer be balancedafter enameling, so that they become limited insofar as the rpms thatcan be achieved are concerned. That is, one cannot use as high an rpm asin the case of stirring elements which do not need to be corrosionresistant and therefore are not enameled. Then, too, it was found thatin the case of the known designs, the enameled tanks had a considerablylonger service life than the enameled stirring elements, so that it wasnecessary to provide the tanks during their service life with severalnew stirring elements. This, of course, represents a considerablefinancial burden.

From German Utility Patent No. 1,847,818, published on Mar. 8, 1962, acirculation mixing head for motor driven mixing arrangements for liquidmedii made up of welded together parts of plastic, such aspolyvinylchloride, is known. The purpose of using plastic was the costsavings realized over the use of high grade metal. However, there is noindication that the peculiarities of the problem, the solution to whichgives rise to the present invention, were ever taken into consideration;and especially not the requirements to ensure sealing under thermalstrain, to avoid welding in the filling area and the possibility ofusing polytetrafluoroethylene.

OBJECTS AND SUMMARY OF THE INVENTION It is, therefore, a general objectof the present invention to overcome the adverse aspects associated withenameling the stirring elements of a stirrer utilized for stirring acorrosive material.

It is, therefore, a more specific object of the present invention toprovide corrosion resistant stirring elements for the chemical industrywhich operate in high pressure tanks with a corrosion resistant lining,such as 2 glass, which have a service life corresponding to the servicelife of the tanks with which they are used.

It is a related object of the present invention to provide a corrosionresistant stirring element utilizing the known advantageouscharacteristics of plastics and in particular the advantageouscharacteristics of polytetrafluoroethylene (PTFE) which has been foundto have a resistance to chemical attack and a temperature resistancecomparable with corresponding characteristics of enamel.

These and other objects of the present invention are accomplished bycovering that portion of the stirring shaft which contacts the corrosivecharge material with a plastic cover, and by mounting the plastic coverso that one end is firmly secured to the stirring element and the otherend is firmly secured to the stirring shaft.

Before proceeding to a detailed discussion of the present invention, itshould be noted that attempts have already been made to make steel partscorrosion resistant by coating them with plastic in its liquid state.Protective coatings made from liquid plastic, however, always possesspores which are permeated by chemicals, so that after a short time themetal parts thus coated are attacked.

In addition, welded joints in plastic materials, including PTFE, canonly with difficulty be produced in a sufficiently pore-free manner foruse in the chemical industry for immersed parts, with the result thatthe problem also called for the avoidance of welded joints in theplastic material, at least in the area of immersion.

Another fact to consider is that PTFE, which is the most desirableplastic to use, isinclined to run (socalled cold flow). It can thereforebe subjected only to low specific surface pressure, or it has to beencased. Also, it has no noteworthy elasticity. Therefore, it does notbehave similarly to rubber with the result that it does notautomatically return to its previous shape after the cause for itschanged shape is eliminated.

The solution to this problem is to be found in the present invention inthe fact that at least that part of the metallic stirring shaft whichcomes into contact with the charge material in the tank is covered by aplastic cover which is driven with the stirrer. The plastic coverrepresents a prefabricated special construction element, preferably anextruded pipe element which has one of its ends firmly secured to thestirring element of the stirrer and the other of its ends firmly securedto the stirring shaft of the stirrer.

One development according to the present invention lies in the fact thatmany plastics and particularly PTFE can be easily machined, so that theplastic cover can be configured into any desired shape, and particularlyinto those shapes which are effective for desirable flow and stirringcharacteristics. Thus, the need for enameling in order to shape thestirring elements is eliminated. Also, the advantageous characteristicsof PTFE are particularly realized when used to fabricate the plasticcover. Like other plastics, however, PTFE contracts and expands independence upon temperature fluctuations, and it does so by much greateramounts than metals, and therefore also by a much greater amount than ametal such as steel, which is used for the stirring shafts for which theplastic-covering is to be used.

According to a further development of the present invention, the plasticcover is made particularly resilient over at least a part of its lengthbetween its secured ends, so that it is capable of returning to itsinitial shape even when it changes its shape as a result of changes in 3temperature. In such an embodiment, the cover has a bellows shape overpart of its length near that end which is secured to the stirring shaft.

According to another further development of the present invention, theplastic cover can be bulged out freely between its secured ends.

According to a preferred embodiment, the single piece stirring elementof the stirrer and the plastic cover for the stirring shaft both consistof PTFE.

A further development of the present invention lies in the fact thatbetween a connecting rod clamping together the stirring element and thestirring shaft, and one of the elements clamped together by it, amechanism, preferably a spring, resiliently limiting the transferableclamping power has been inserted. This will counteract the cold flow ofsubstance like PTFE.

Other further developments and details of embodiments of the inventionwill unfold from the embodiment described in the following paragraphs byway of example and in connection with the claims and the drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates in elevation apartial cross-sectional view of a stirrer according to the presentinvention, with the outside ends of the stirring arms cut away;

FIG. 2 illustrates a partial top view of the stirring element of FIG. 1;

FIG. 3 is a view taken along the lines 3-3 of FIG. 2;

FIG. 4 is a top plan view of the mounting plate for mounting the tie rodand consequently the stirring shaft to the stirring element; and

FIG. 5 is a top plan view illustrating one set of relative positions ofadjustable abutment elements which are mounted to the mounting plate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now more specificallyto the drawing, FIG. 1 shows a stirring element 1 fabricated as aone-piece element of PTFE with two stirring arms 2 extending radiallyoutward from a central hub 4. The arms 2 have, as best seen in FIG. 3, across section similar to propeller blades. This cross section isproduced by a chipping treatment of a starting piece which has a squareprofile.

The stirring element 1 with its hub 4 is attached to the lower end of astirring shaft 7 in a manner which will be explained in more detail inthe following paragraphs. The stirring shaft 7 is developed as a hollowshaft and is made of a metal, such as steel, preferably high gradesteel.

The shaft 7 comprises an upper portion 7, an intermediate metallic weldmember 24 and an end seal 25. The lower end of the upper portion 7' hasdefined therein a recess 7" and the upper end of the end seal 25 hasdefined therein a recess 25. Both these recesses provide mounting areasfor receiving respective ends of the weld member 24 for welding to theupper portion 7' and the end seal 25, respectively. Both the upperportion 7 and the end seal 25 are provided with shoulders which form alimit to the recesses 7" and 25, respectively, on one end, and definefrusto-conical surfaces 22 and 19, respectively. In addition, the endseal is also provided with a torque transmitting extension 8 which has asquare configuration.

The shaft 7 is constructed to have a larger diameter in its lower areathat is, in the area thereof which comes into contact with the materialwith which the tank is charged, than in its area which does not contactthe charge material. The larger diameter area is essentially formed bythe weld member 24 which comprises a piece of pipe. In the area whichcomes into contact with the charge material, which for convenience willbe designated as area A, the stirring shaft 7 is covered by a plasticcover 18.

The hub 4 defines a bore which is configured to have a first portion 4which defines a conical surface 4", a second portion 5 in the form of asquare recess and a third portion 6 in the form of a cylindricalcounterbore which defines an abutment surface 17.

The cover 18 is a prefabricated, specially constructed element, that is,an element which is extruded in pipe form of PTFE. The upper end of thecover 18 is pressed tightly against the conical surface 22 by means of ascrew cap 21, which is provided with a surface 21' and screw threads 20for this purpose. The screw cap 21 is screwed onto the upper portion 7of the stirring shaft 7. The lower end of the cover 18 is pressedtightly against the conical surface 4" of the hub portion 4', whichgenerally corresponds to the surface 19. In the area between its tightlyattached ends, that is, in area A, the cover 18 may be shrunk on.Alternatively, the cover 18 could also loosely surround the shaft 7 inthe area A, so that it can expand or bulge out unimpeded with increasingtemperatures, and can contract again in the case of shrinkage due to adrop in temperature. Yet another alternative exists in that the cover 18can also be made resilient, by being developed in the manner of bellowsnear its upper secured end. The bellows can be produced originally as aseparate part and can be welded together with the downwardly succeedingpart of the cover, especially whenever the place of welding is above thefilling level of the vessel.

The square recess of the second portion 5 has been so formed in order toengageably receive therein the square torque transmitting extension 8.As a result of this engagement, the stirring element 1 is driven by thestirring shaft 7.

To mount the stirring element 1 to the shaft 7 so that the shaft 7 candrive the stirring element 1, a tie rod 13, a plate 9 and a plurality ofadjustable abutment ele' ments 10 are provided. The tie rod 13 passesthrough a central bore 7' in the shaft 7 and extends outwardly from theend seal 25. The outwardly extending end of the tie rod 13 is providedwith external threads which threadedly engage the tie rod 13 within acentral bore 12 of the plate 9.

Referring to FIG. 4 it can be seen that the plate 9 is square in shapeso that it can be engageable within the square shaped second portion 5of the bore 4. Surrounding the bore 12 are bolt receiving bores 12'.Bolts 11 are provided with a head portion 11 which contains a hexagonalor polyhedral recess and a shank portion 11". An abutment element head11" is provided for each bolt 11 on which a respective abutment element10 is mounted. The head 11" may be a hexagonal nut or the like.

As shown in FIG. 5, the abutment elements 10 have longitudinal slots 26therein through which the bolts 11 pass. As a result of the slots 26,the abutment elements 10 can be mounted by the bolts 1 1 to the plate 9so that the plate 9 can be inserted into the recess formed by the secondportion 5 of the bore 4 to the position shown in FIG. 1. The abutmentelements 10 are then swiveled into the counterbore 6. Thereafter thebolts are held in turn by a tool (not shown), which engages within thehexagonal recesses, while the nuts 11" are rotated by a tool (not shown)which passes through the bore 12. This causes the abutment elements tobe maintained in their swiveled position in the counterbore 6 so thatthey are firmly engaged against the abutment surface 17. Thereafter, thetie rod 13 is threadedly engaged within the bore 12 of the plate 9 toform the assembly.

A compression spring 16 is inserted between a head portion 15 of the rod13 and an internal shoulder 23 in the upper portion 7' of the stirringshaft 7. The spring 16 is dimensioned such that its windings will notfit against each other, that is, the spring will not completely close,even if the tie rod 13 is tightened to a point where it clamps thestirring shaft 7 tightly together with the stirring element 1. Thespring 16 is selected so that the clamping force will not produce toohigh a specific surface pressure between the abutment element 10 and theabutment surface 17, so that therefore, a cold flow of the PTFE of thestirring element 1 will be avoided.

I claim:

1. In a stirrer for stirring corrosive charges in corrosion resistant,high pressure reaction tanks, such as glass-lined tanks, the stirrerincluding a metallic stirring shaft and a plastic stirring elementattached to one end of the stirring shaft, said stirring element beingdriven by said stirring shaft, the stirring element being fabricated asa one-piece element having a plurality of arms, the improvementcomprising: a plastic cover covering a portion of the stirring shaftwhich contacts the corrosive charge, said cover having two end portionswith one end portion being sealingly secured to the stirring element andthe other end portion being sealingly secured to the stirring shaftwherein said cover has an intermediate portion between its secured endportions which is movable relative to said end portions, and whereinsaid cover is fabricated so that at least part of its length between itssecured ends is resilient and thereby capable of returning to itsinitial shape as a result of temperature changes.

2. In a stirrer for stirring corrosive charges in corrosion resistant,high pressure reaction tanks, such as glass-lined tanks, the stirrerincluding a metallic stirring shaft and a plastic stirring elementattached to one end of the stirring shaft, the stirring element beingfabricated as a one-piece element having a plurality of arms, theimprovement comprising: a plastic cover covering a portion of thestirring shaft which contacts the corrosive charge, said cover beingfabricated as an extruded pipe and having two end portions with one endportion being sealingly secured to the stirring element and the otherend portion being sealingly secured to the stirring shaft.

3. In a stirrer for stirring corrosive charges in corrosion resistant,high pressure reaction tanks, such as glass-lined tanks, the stirrerincluding a metallic stirring shaft having a frusto-conical portion anda plastic stirring element attached to one end of the stirring shaft,the stirring element being fabricated as a one-piece element having aplurality of arms, the improvement comprising: a plastic cover coveringa portion of the stirring shaft which contacts the corrosive charge,said cover having two end portions with one end portion being sealinglysecured to the stirring element and the other end portion beingsealingly secured to the stirring shaft; a screw cap which threadedlyengages said stirring shaft, said screw cap having a surface which isgenerally parellel to the outer surface of said frusto-conical portion,said threaded engagement being such that said one end portion of saidcover can be received and secured between said surfacesv 4. In a stirrerfor stirring corrosive charges in corrosion resistant, high pressurereaction tanks, such as glass-lined tanks, the stirrer including ametallic stirring shaft and a plastic stirring element attached to oneend of the stirring shaft, the stirring element being fabricated as aone-piece element having a plurality of arms, the improvementcomprising: a plastic cover covering a portion of the stirring shaftwhich contacts the corrosive charge, said cover having two end portionswith one end portion being sealingly secured to the stirring element andthe other end portion being sealingly secured t0 the stirring shaft,wherein the diameter of said stirring shaft along the portion thereofwhich is covered by said cover and is in the area of the corrosivecharge is larger than the diameter of said stirring shaft along theportion thereof which is not covered by said cover.

5. A stirrer as defined in claim 4, wherein the larger diameter portionof said stirring shaft includes an end seal and a metallic weld member,said end seal having a recess for receiving and having welded thereinone end of said weld member, and wherein said end seal includes afrusto-conical sealing portion and a torque transmitting extension.

6. In a stirrer for stirring corrosive charges in corrosion resistant,high pressure reaction tanks, such as glass-lined tanks, the stirrerincluding a metallic stirring shaft and a plastic stirring elementattached to one end of the stirring shaft, the stirring element beingfabricated as a one-piece element having a plurality of arms, theimprovement comprising: a plastic cover covering a portion of thestirring shaft which contacts the corrosive charge, said cover havingtwo end portions with one end portions with one end portion beingsealingly secured to the stirring element and the other end portionbeing sealingly secured to the stirring shaft; a tie rod; and a holdingmechanism, said tie rod being adapted to attach said stirring shaft tosaid stirring element and said holding mechanism being adapted to effectsaid attachement, wherein said stirring element includes a bore defininghub portion at its center, and said stirring shaft includes an end sealhaving a frustoconical sealing portion and a torque transmittingextension, and wherein said bore is configured to define a first portionhaving a surface which is generally parallel to the outer surface ofsaid frusto-conical portion and defining a receiving space for saidfrusto-conical portion, a second portion defining a receiving space forsaid torque transmitting extension and a third portion defining anabutment surface for said holding mechanism.

7. A stirrer as defined in claim 6, wherein said holding mechanismcomprises: a mounting plate having a central threaded bore within whichsaid tie rod is threadedly received and a plurality of bolt receivingbores, said plate being adapted for reception within said second boreportion; and a plurality of adjustable abutment elements, each beingrotatably mounted by a bolt to said plate at a respective one of saidbolt receiving bores, and each being positioned relative to said plateso that said plate can be received into said second bore portion andthereafter each is positioned within said third bore portion and inengagement with said abutment surface.

8. In a stirrer for stirring corrosive charges in corrosion resistant,high pressure reaction tanks, such as glass-lined tanks, the stirrerincluding a hollow metallic stirring shaft and a plastic stirringelement attached to one end of the stirring shaft, the stirring elementbeing fabricated as a one-piece element having a hub portion and aplurality of arms, the improvement comprising: a plastic cover coveringa portion of the stirring shaft which contacts the corrosive charge,said cover having two end portions with one end portion being sealinglysecured to the stirring element and the other end portion beingsealingly secured to the stirring shaft; a tie rod which passes throughsaid hollow stirring shaft; and means mounting said tie rod within saidhub portion to said stirring element to thereby clamp together saidcover, said stirring shaft and said stirring element and effect one ofthe secured ends of said cover.

9. A stirrer as defined in claim 8, wherein the improvement furthercomprises: resilient means mounted between said tie rod and saidstirring element thereby resiliently limiting the transferable clampingforce.

10. A stirrer as defined in claim 9, wherein said resilient means is aspring.

11. A stirrer as defined in claim 8, wherein said hub portion defines abore and said stirring shaft includes an end seal having afrusto-conical sealing portion and a torque transmitting extension, andwherein said mounting means comprises: a holding mechanism which isadapted to mount said tie rod to said stirring element within the boreof said hub portion, said bore being configured to define a firstportion having a surface which is generally parallel to the outersurface of said frusto-conical portion and defining a receiving spacefor said frusto-conical portion, a second portion defining a receivingspace for said torque transmitting extension and a third portiondefining an abutment surface for said holding mechanism.

12. A stirrer as defined in claim 11, wherein said holding mechanismcomprises: a mounting plate having a central threaded bore within whichsaid tie rod is threadedly received and a plurality of bolt receivingbores, said plate being adapted for reception within said second boreportion; and a plurality of adjustable abutment-elements, each beingrotatably mounted by a bolt to said plate at a respective one of saidbolt receiving bores, and each being positioned relative to said 8 plateso that said plate can be received into said second bore portion andthereafter each is positioned within said third bore portion and inengagement with said abutment surface.

13. A stirrer as defined in claim 8, wherein the improvement furthercomprises: resilient means mounted between said tie rod and saidstirring shaft thereby resiliently limiting the transferable clampingforce.

14. A stirrer as defined in claim 13, wherein said resilient means is aspring.

15. A stirrer as defined in claim 13, wherein said tie rod is providedwith a head and said stirring shaft is provided with a shoulder, andwherein said resilient means is mounted between said head and saidshoulder.

16. In a stirrer for stirring corrosive charges in corrosion resistant,high pressure reaction tanks, such as glass-lined tanks, the stirrerincluding a metallic stirring shaft and a plastic stirring elementattached to one end of the stirring shaft, the stirring element beingfabricated as a one-piece element having a plurality of arms, theimprovement comprising: a plastic cover covering a portion of thestirring shaft which contacts the corrosive charge, said cover havingtwo end portions with one end portion being sealingly secured to thestirring element and the other portion being sealingly secured to thestirring shaft; a tie rod for clampably mounting said stirring shaft tosaid stirring element; and resilient means mounted between said tie rodand said stirring shaft thereby resiliently limiting the transferableclamping force.

17. A stirrer as defined in claim 16, wherein said resilient means is aspring.

18. A stirrer as defined in claim 16, wherein said cover has anintermediate portion between its secured end portions which is movablerelative to said end portions.

19. A stirrer as defined in claim 18, wherein said cover is fabricatedso that at least part of its length between its secured ends isresilient and thereby capable of returning to its initial shape when itchanges its shape as a result of temperature changes.

20. A stirrer as defined in claim 16, wherein both said stirring elementand said cover consist of polytetrafluoroethylene.

1. IN A STIRRING CORROSIVE CHARGES IN CORROSION RESISTANT, HIGH PRESSUREREACTION TANKS, SUCH AS GLASS-LINED TANKS, THE STIRRER INCLUDING AMETALLIC STIRRING SHAFT AND A PLASTIC STIRRING ELEMENT ATTACHED TO ONEEND OF THE STIRRING SHAFT, SAID STIRRING ELEMENT BEING DRIVEN BY SAIDSTIRRING SHAFT, THE STIRRING ELEMENT BEING FAFABRICATED AS A ONE-PIECEELEMENT HAVING A PLURALITY OF ARMS, THE IMPROVEMENT COMPRISING: APLASTIC COVER COVERING A PORTION OF THE STIRRING SHAFT WHICH CONTACTSTHE CORROSIVE CHARGE, SAID COVER HAVING TWO END PORTIONS WITH ONE ENDPORTION BEING SEALINGLY SECURED TO THE STIRRING ELEMENT AND THE OTHEREND PORTION BEING SEALINGLY SECURED TO THE STIRRING SHAFT WHEREIN SAIDCOVER HAS AN INTERMEDIATE PORTION BETWEEN ITS SECURED END PORTIONS WHICHIS MOVABLE RELATIVE TO SAID END PORTIONS, AND WHEREIN SAID COVER ISFABRICATED SO THAT AT LEAST PART OF ITS LENGTH BETWEEN ITS SECURED ENDSIS RESILIENT
 2. In a stirrer for stirring corrosive charges in corrosionresistant, high pressure reaction tanks, such as glass-lined tanks, thestirrer including a metallic stirring shaft and a plastic stirringelement attached to one end of the stirring shaft, the stirring elementbeing fabricated as a one-piece element having a plurality of arms, theimprovement comprising: a plastic cover covering a portion of thestirring shaft which contacts the corrosive charge, said cover beingfabricated as an extruded pipe and having two end portions with one endportion being sealingly secured to the stirring element and the otherend portion being sealingly secured to the stirring shaft.
 3. In astirrer for stirring corrosive charges in corrosion resistant, highpressure reaction tanks, such as glass-lined tanks, the stirrerincluding a metallic stirring shaft having a frusto-conical portion anda plastic stirring element attached to one end of the stirring shaft,the stirring element being fabricated as a one-piece element having aplurality of arms, the improvement comprising: a plastic cover coveringa portion of the stirring shaft which contacts the corrosive charge,said cover having two end portions with one end portion being sealinglysecured to the stirring element and the other end portion beingsealingly secured to the stirring shaft; a screw cap which threadedlyengages said stirring shaft, said screw cap having a surface which isgenerally parellel to the outer surface of said frusto-conical portion,said threaded engagement being such that said one end portion of saidcover can be received and secured between said surfaces.
 4. In a stirrerfor stirring corrosive charges in corrosion resistant, high pressurereaction tanks, such as glass-lined tanks, the stirrer including ametallic stirring shaft and a plastic stirring element attached to oneend of the stirring shaft, the stirring element being fabricated as aone-piece element having a plurality of arms, the improvementcomprising: a plastic cover covering a portion of the stirring shaftwhich contacts the corrosive charge, said cover having two end portionswith one end portion being sealingly secured to the stirring element andthe other end portion being sealingly secured to the stirring shaft,wherein the diameter of said stirring shaft along the portion thereofwhich is covered by said cover and is in the area of the corrosivecharge is larger than the diameter of said stirring shaft along theportion thereof which is not covered by said cover.
 5. A stirrer asdefined in claim 4, wherein the larger diameter portion of said stirringshaft includes an end seal and a metallic weld member, said end sealhaving a recess for receiving and having welded therein one end of saidweld member, and wherein said end seal includes a frusto-conical sealingportion and a torque transmitting extension.
 6. In a stirrer forstirring corrosive charges in corrosion reSistant, high pressurereaction tanks, such as glass-lined tanks, the stirrer including ametallic stirring shaft and a plastic stirring element attached to oneend of the stirring shaft, the stirring element being fabricated as aone-piece element having a plurality of arms, the improvementcomprising: a plastic cover covering a portion of the stirring shaftwhich contacts the corrosive charge, said cover having two end portionswith one end portions with one end portion being sealingly secured tothe stirring element and the other end portion being sealingly securedto the stirring shaft; a tie rod; and a holding mechanism, said tie rodbeing adapted to attach said stirring shaft to said stirring element andsaid holding mechanism being adapted to effect said attachement, whereinsaid stirring element includes a bore defining hub portion at itscenter, and said stirring shaft includes an end seal having afrusto-conical sealing portion and a torque transmitting extension, andwherein said bore is configured to define a first portion having asurface which is generally parallel to the outer surface of saidfrusto-conical portion and defining a receiving space for saidfrusto-conical portion, a second portion defining a receiving space forsaid torque transmitting extension and a third portion defining anabutment surface for said holding mechanism.
 7. A stirrer as defined inclaim 6, wherein said holding mechanism comprises: a mounting platehaving a central threaded bore within which said tie rod is threadedlyreceived and a plurality of bolt receiving bores, said plate beingadapted for reception within said second bore portion; and a pluralityof adjustable abutment elements, each being rotatably mounted by a boltto said plate at a respective one of said bolt receiving bores, and eachbeing positioned relative to said plate so that said plate can bereceived into said second bore portion and thereafter each is positionedwithin said third bore portion and in engagement with said abutmentsurface.
 8. In a stirrer for stirring corrosive charges in corrosionresistant, high pressure reaction tanks, such as glass-lined tanks, thestirrer including a hollow metallic stirring shaft and a plasticstirring element attached to one end of the stirring shaft, the stirringelement being fabricated as a one-piece element having a hub portion anda plurality of arms, the improvement comprising: a plastic covercovering a portion of the stirring shaft which contacts the corrosivecharge, said cover having two end portions with one end portion beingsealingly secured to the stirring element and the other end portionbeing sealingly secured to the stirring shaft; a tie rod which passesthrough said hollow stirring shaft; and means mounting said tie rodwithin said hub portion to said stirring element to thereby clamptogether said cover, said stirring shaft and said stirring element andeffect one of the secured ends of said cover.
 9. A stirrer as defined inclaim 8, wherein the improvement further comprises: resilient meansmounted between said tie rod and said stirring element therebyresiliently limiting the transferable clamping force.
 10. A stirrer asdefined in claim 9, wherein said resilient means is a spring.
 11. Astirrer as defined in claim 8, wherein said hub portion defines a boreand said stirring shaft includes an end seal having a frusto-conicalsealing portion and a torque transmitting extension, and wherein saidmounting means comprises: a holding mechanism which is adapted to mountsaid tie rod to said stirring element within the bore of said hubportion, said bore being configured to define a first portion having asurface which is generally parallel to the outer surface of saidfrusto-conical portion and defining a receiving space for saidfrusto-conical portion, a second portion defining a receiving space forsaid torque transmitting extension and a third portion defining anabutment surface for said holding mechanism.
 12. A stirrer as defined incLaim 11, wherein said holding mechanism comprises: a mounting platehaving a central threaded bore within which said tie rod is threadedlyreceived and a plurality of bolt receiving bores, said plate beingadapted for reception within said second bore portion; and a pluralityof adjustable abutment elements, each being rotatably mounted by a boltto said plate at a respective one of said bolt receiving bores, and eachbeing positioned relative to said plate so that said plate can bereceived into said second bore portion and thereafter each is positionedwithin said third bore portion and in engagement with said abutmentsurface.
 13. A stirrer as defined in claim 8, wherein the improvementfurther comprises: resilient means mounted between said tie rod and saidstirring shaft thereby resiliently limiting the transferable clampingforce.
 14. A stirrer as defined in claim 13, wherein said resilientmeans is a spring.
 15. A stirrer as defined in claim 13, wherein saidtie rod is provided with a head and said stirring shaft is provided witha shoulder, and wherein said resilient means is mounted between saidhead and said shoulder.
 16. In a stirrer for stirring corrosive chargesin corrosion resistant, high pressure reaction tanks, such asglass-lined tanks, the stirrer including a metallic stirring shaft and aplastic stirring element attached to one end of the stirring shaft, thestirring element being fabricated as a one-piece element having aplurality of arms, the improvement comprising: a plastic cover coveringa portion of the stirring shaft which contacts the corrosive charge,said cover having two end portions with one end portion being sealinglysecured to the stirring element and the other portion being sealinglysecured to the stirring shaft; a tie rod for clampably mounting saidstirring shaft to said stirring element; and resilient means mountedbetween said tie rod and said stirring shaft thereby resilientlylimiting the transferable clamping force.
 17. A stirrer as defined inclaim 16, wherein said resilient means is a spring.
 18. A stirrer asdefined in claim 16, wherein said cover has an intermediate portionbetween its secured end portions which is movable relative to said endportions.
 19. A stirrer as defined in claim 18, wherein said cover isfabricated so that at least part of its length between its secured endsis resilient and thereby capable of returning to its initial shape whenit changes its shape as a result of temperature changes.
 20. A stirreras defined in claim 16, wherein both said stirring element and saidcover consist of polytetrafluoroethylene.